1. Field of the Invention
The present invention relates generally to an apparatus and method for a Multicast and Broadcast Service (MBS) in a Broadband Wireless Access (BWA) system, and in particular, to an apparatus and method for simultaneously providing an MBS and a unicast service in a BWA system.
2. Description of the Related Art
As generally known in the art, communication systems have been primarily developed for voice communication services, but they are also evolving to provide data services and various multimedia services. However, conventional communication systems, which are mainly directed to providing voice communication services, still have a narrow data transmission bandwidth and require a high subscription fee. For these reasons, they cannot satisfy diversified user demands. Furthermore, in line with rapid development in the communication industry and ever-increasing demands on Internet services, it is important to provide communication systems capable of providing Internet services efficiently. As a result of these trends, BWA systems having a bandwidth large enough to both satisfy the increasing user demands and provide efficient Internet services have been proposed.
In addition to providing voice communication services, BWA systems aim at supporting a variety of low-speed and high-speed data services and multimedia application services (e.g., high-quality moving pictures) in combination. Based on wireless media using a broadband of 2 GHz (GigaHertz), 5 GHz, 26 GHz, or 60 GHz, BWA systems are able to access a Public Switched Telephone Network (PSTN), Public Switched Data Network (PSDN), Internet network, International Mobile Telecommunications-2000 (IMT-2000) network, and Asynchronous Transfer Mode (ATM) network in a mobile or stationary environment. In other words, BWA systems can support a channel transmission rate of at least 2 Mbps (Megabits per second). BWA systems may be classified into broadband wireless local loops, broadband mobile access networks, and high-speed wireless Local Area Networks (LANs) according to the terminal's mobility (stationary or mobile), communication environment (indoor or outdoor), and channel transmission rate.
The standardization of wireless access schemes of BWA systems is being conducted by Institute of Electrical and Electronics Engineers (IEEE), which is one of the international standardization organizations, particularly by the IEEE 802.16 standardization group.
Compared to conventional wireless communication systems for voice communication services, IEEE 802.16 communication systems have a larger data transmission bandwidth. Therefore, they can transmit more data for a limited period of time and share all user channels (or resources) for efficient channel utilization. Also, since Quality of Service (QoS) features are guaranteed, users can be provided with various services of different qualities depending on the characteristics of services.
Examples of principal services of the BWA systems are Internet services, Voice over Internet Protocol (VoIP) services, and nonreal-time streaming services. Recently, a Multicast and Broadcast Service (MBS) has emerged as a new real-time broadcast service. An MBS can simultaneously provide more channels while supporting mobility like a Digital Multimedia Broadcasting (DMB) service.
When an MBS is provided in an BWA system, subscribers receiving only a unicast service may coexist with subscribers receiving only the MBS. However, when there are user terminals that are in an awake (or active) mode while receiving only an MBS, a restriction may be imposed on the subscriber capacity available in the system. Furthermore, when an awake-mode MBS-receiving user terminal moves to another base station, the broadcast service may be interrupted due to a handover. This will be described in detail below.
FIG. 1 shows a state transition diagram of a user terminal in a conventional BWA system.
In FIG. 1, the state of the user terminal includes a null mode 101, an awake mode 103, a sleep mode 105, and an idle mode 107. The state transition described below can be applied to a base station as well as to the user terminal.
The null mode 101 is a state where the user terminal is powered off. When the user terminal is powered on, the user terminal establishes a wireless connection via Dynamic Service Addition (DAS) procedure and to this end, transitions to the awake mode 103. The awake mode 103 is a state where a base station applies an AMC scheme to uplink (or downlink) data of the user terminal with dynamic consideration of the channel condition.
After transition to the awake mode 103, the user terminal desiring to receive MBS reception requests broadcast contents from an MBS controller and receives broadcast contents from the MBS controller. Unless the user terminates the MBS, the user terminal operates in the awake mode 103 due to the frequent Media Access Control (MAC) message transmissions (ex: DSx) for creating and deleting connections between a Base Station (BS), where the Mobile Station (MS) in the awake or sleep mode experiences a handover procedure for mobility and load balancing.
If there is no traffic for a particular time (e.g., a sleep timer) after termination of the MBS, the user terminal transitions from the awake mode 103 to the sleep mode 105. After transition to the sleep mode 105, the user terminal checks whether there is a traffic until the expiration of an idle timer. Upon detection of the traffic, the user terminal transitions to the awake mode 103 to transmit/receive the traffic. Herein, whether there is a traffic may be checked by receiving a traffic indication (TRF-IND) message from the system during a listening period of the sleep mode.
If there is no traffic until the expiration of the idle timer, the user terminal transitions to the awake mode 103 to exchange messages with a base station and then transitions to the idle mode 107. That is, the user terminal transitions to the awake mode 103 to release a wireless connection with the base station and then transitions to the idle mode 107. The idle mode 107 is the state where the wireless connection is released but Service Flow (SF) information is retained. After transition to the idle mode 107, the user terminal checks whether there is an incoming traffic at particular periods. Upon detection of an incoming traffic, the user terminal transitions to the awake mode 103 to transmit/receive traffics. Incoming traffic may be checked by receiving a paging advertisement (PAG-ADV) message from the system during a listening period of the idle mode.
As described above, according to the standards, the sleep timer (optional) and the idle timer (mandatory) are managed by both of the user terminal and the system (i.e., the base station). If the timer expires, one of the user terminal and the base station requests a mode transition to the counterpart. Thereafter, if the counterpart makes an acknowledgement response, both of the user terminal and the base station transition to the same mode. On the other hand, if the opponent does not make an acknowledgement response, the previous state is maintained. Also, for transition from the sleep mode to the idle mode, the awake mode intervenes between the sleep mode and the idle mode. If an error occurs in the transition process for certain reason, a transition can be made to the null mode.
As described above, the conventional user terminal receiving the MBS always operates in the awake (or active) mode. In general, because at least one or more content frames are transmitted every 256 frames (i.e., 1.28 sec) in the MBS and a transmission interval is much smaller in a target broadcast channel of 64 Kbps (Kilobits per second) or more, the conventional MBS terminal cannot transition to the idle mode or the sleep mode. The following problems arise because the conventional MBS terminal always operates in the awake mode.
Firstly, there is a restriction in the subscriber capacity available in the system. In general, the BWA system restricts the maximum available number of awake-mode terminals due to a restriction in allocation of a Channel Quality Indicator (CQI) channel. The MBS can perform a scheduling operation without consideration of the wireless conditions of a user terminal or a user-terminal group. In this case, the MBS does not use an Adaptive Modulation and Coding (AMC) scheme. That is, the MBS does not need resource scheduling. Thus, the MBS need not allocate a CQI channel. However, because user terminals receiving only the MBS also operate in the awake mode, the number of awake-mode user terminals may be saturated due to the user terminals receiving only the MBS. For example, an access attempted by a new user terminal may be rejected due to the user terminals receiving only the MBS, though the system can accommodate the new user terminal.
Secondly, a handover procedure is executed when a user terminal receiving only the MBS moves between base stations, though the handover procedure is unnecessary. The execution of the unnecessary handover procedure causes a waste of air resource. In the worst case, the broadcast service may be interrupted because an initial network entry procedure is executed when the handover procedure fails.
As described above, various problems arise because user terminals receiving only an MBS operate in an awake mode.